The build up and subsequent discharge of static charges from a person to electrical equipment and component parts can cause complete failure of a component, or worse, degradation which might not be discoverable until the component is operated for a time in a system and then fails. The advent of large scale integration of electrical components has resulted in even greater sensitivity to electro-static discharge ("ESD"). ESD, therefore, detrimentally affects quality, reliability, maintainability, and repairability. One estimate of the cost of ESD-caused circuit board and semiconductor handling and equipment disruption and damage is as high as $10 billion per year. Consequently, many devices have been employed to combat ESD in manufacturing and other facilities were electronic components are exposed, assembled, and tested. Hence the competition in the marketplace is fierce.
In the electronic manufacturing environment, these are a plethora of devices used to combat ESD.
There are antistatic smocks and trousers as well as grounding wrist and shoe straps worn by engineers and technicians working on sensitive components. Other devices include air ionizers, humidity measurement and control devices, and protective "pink-poly" and so called "Faraday cage" coverings for electronic components. There are also antistatic coatings and various antistatic sprays. All these devices aid in the reduction of possible ESD damage.
Static charges, however, are primarily generated when people walk across the floor. For example, as much as 15,000 volts can be generated by walking across regular nylon carpet. As much as 5,000 volts can be generated while walking across a linoleum floor. Since a MOSFET device is susceptible to ESD damage in the range of 100-200 volts, a person walking across these types of floorings may seriously damage these and similar type electronic components. Therefore, it has been determined that a holistic approach to ESD control includes controlling static electricity at its prime source--the floor. This is because air ionizers and wrist straps, although effective, require monitoring for proper use and are mainly localized work station solutions to the ESD problem. By reducing triboelectric generation and accumulation of electrostatic charges, and by dissipating any accumulated charges before personnel arrive at the workstation, ESD-protective floor materials find application throughout the electronic manufacturing and testing environment. ESD controlled flooring can be broken down into two categories: Antistatic flooring, and conductive flooring.
The term "antistatic" means the flooring itself resists triboelectric charging. Therefore, antistatic flooring would be nonconductive (having a resistance of between 10.sup.12 and 10.sup.10 ohms). While flooring that is only antistatic does not contribute to the ESD problem it does not clearly solve the problem either. Charges can accumulate on a person standing on the floor by other means, such as by coming into contact with paper that has become charged after passing through a laser printer. Or, the person could have picked up charges while walking across other non-ESD controlled surfaces. Since true antistatic floors are not conductive, these accumulated charges will not dissipate until, perhaps, the person touches an ESD sensitive component.
Conductive floors, of course, have a low resistance which does help in bleeding off any accumulated charges.
Therefore, it has long been recognized that proper flooring should have both antistatic and conductive properties. ESD floor coverings should both inhibit charge generation and quickly bleed off any accumulated charges before a person can "zap" any components.
Conductive rubber floors, conductive epoxy floors, conductive vinyl floor tiles secured with conductive epoxy, and various conductive waxes have been used to combat ESD. The acoustic, comfort, and ergonomic properties of these materials, however, are less than desirable. For example, high noise levels inherent in facilities utilizing vinyl floor tiles affect concentration and interfere with certain precise measurements.
Such floor coverings also detract from the ability to hear abnormal sounds in various equipment during testing and evaluation. Also, these floorings can cause echos and background noise which are distracting to human concentration. For example, hard surface flooring may have a noise reduction coefficiency upwards of 10 times louder than that of carpet flooring. Additionally, walking or standing on hard floors for long periods of time adversely affects human comfort. It has been determined that decreases in human comfort decrease productivity and increases absenteeism. Finally, most hard floorings often pose a serious slip hazard.
Conductive vinyl tiles are often difficult and expensive to maintain. If left unsealed, the porous surface stains easily and is difficult to keep clean. Moreover, if accidentally waxed by maintenance personnel unaware of the conductive properties of the tiles, the tiles are rendered nonconductive. Often, special signs must be included directing maintenance personnel not to wax the tiles with commercial waxes. One manufacturer goes so far as to include special tiles with die cut insert letters of contrasting color reading "CONDUCTIVE FLOOR DO NOT WAX." Furthermore, damaged conductive vinyl tiles require special tools and techniques for replacement. Finally, dust and other contaminants present on vinyl tiles interfere with charge dissipation.
Finally, the configuration of many modern facilities is often in a state of constant flux--remodeling can occur every two or three years or even more frequently. Vinyl tiles may be removed (and sometimes destroyed in the process) while they still have much useful life.
Other types of ESD control surfaces include sealed concrete with conductive paint and epoxy flooring poured over an existing subfloor. But, these floorings have most, if not all, of the limitations of the vinyl floor tiles and waxes discussed above. Conductive waxes often quickly loose their electrical properties, and hence require continuous reapplication which is cost prohibitive in many facilities. The cost of maintaining a hard flooring including waxing, stripping, buffing, and reapplication of the wax can run as high as $2.00 a square foot per year. Therefore, some attempt has been made to develop ESD safe carpet, the maintenance cost of which may be as low as 0.25.cent. a square foot per year.
Commercial antistatic nylon has proven safe for an office-type atmosphere, but as discussed above, built up charges are not dissipated by carpet that is only antistatic. Therefore, it is not suitable for facilities where sensitive components are manufactured or tested. The fibers of these and other carpets treated with coatings to make them somewhat antistatic, but the topical treatment wears off with time. Conductive fibers have been added to the yarn of some carpets, to prevent static generation above the human sensitivity level. These types of carpets, however, are not satisfactory for true ESD control since they do not provide a conductive path to ground.
Broad-loom carpet may have conductive fibers interwoven into the yarn and perhaps conductive primary made of polypropylene with carbon coated nylon fibers or a conductive backing made from bituminous latex impregnated with carbon. Broad-loom carpet by definition, however, is not modular. Therefore, as delineated above, worn out portions are difficult to replace. In addition, broad loom carpet have a tendency to buckle and wrinkle under heavy traffic.
Carpet tiles, which must have sufficient rigidity, durability and strength, require vinyl or urethane layers in addition to the primary backing. Adding carbon to vinyl support layers weakens the strength of the carpet face which affects the physical properties of the carpet. Consequently, making carpet tiles sufficiently conductive is inherently more difficult than making broad-loom carpet conductive. The problem is compounded when the conductive and antistatic carpet tiles must be releasable to facilitate reconfiguration in the dynamic setting of many electronic and manufacturing facilities.
Conductive epoxies and similar adhesives are inherently non-releasable. Also, because of its consistency, conductive epoxy is difficult to apply. Some attempt was made to render known releasable adhesives conductive by adding carbon powders or liquids, but the result was a great loss of tackiness and strength.